Phased array antennas on future Navy ships may be embedded directly onto the ship's structure. By embedding the antennas directly into the ship's structure, it will be possible to make full use of available space on the body of the ship and free up critical space on the ship deck for other critical radars necessary for increasing Navy requirements. However, the ship structure can deform due to mechanical and thermal stresses and these stresses can significantly degrade the antenna radiation pattern.
Ships like DDG-1000, CG(X) and DDG(X) are going to have a large number of cut-outs. Modular antenna array antennas are going to be placed into these cut-outs. Unlike integrated (no cut-out) designs, which are able to maintain structural continuity, the cut-out design suffers from structural inefficiencies such as lack of stiffness, vibration and weight resulting in deformations of the antenna array shape. These deformations cause the antenna pattern to be different than predicted resulting in the degradation of the sidelobe levels (SLL), gain, and beam pointing accuracy. It is important that these errors resulting from the deformations are corrected for so that the antenna array maintains the desired radiation characteristics.
The technique described in McWatters, D.; Freedman, A.; Michel, T.; Cable, V., “Antenna auto-calibration and metrology approach for the AFRL/JPL space based radar,” Radar Conference, 2004. Proceedings of the IEEE, vol., no., pp. 21-26, 26-29 Apr. 2004 uses cameras and receivers placed on a tower to determine the location of the radiating elements in a large array. Triangulation and other metrology techniques are then used to determine any deformations that exist within the aperture, and the phase at each element is then calibrated to correct the pattern shape. This technique allows for in situ calibration, but it requires the installation of a calibration tower for the integration of calibration receivers, transmitters, antennas, and cameras.
The calibration technique introduced in Shipley, C.; Woods, D., “Mutual coupling-based calibration of phased array antennas,” Phased Array Systems and Technology, 2000. Proceedings. 2000 IEEE International Conference on, vol., no., pp. 529-532, 2000 relies on the mutual coupling between elements in the array. In this technique, the calibration signals lie within the operational band of the antenna array. As a result, calibration is not permitted during array operation.
The scheme used in Weijun Yao; Yuanxun Wang; Itoh, T., “A self-calibration antenna array system with moving apertures,” Microwave Symposium Digest, 2003 IEEE MTT-S International, vol. 3, no., pp. 1541-1544 vol. 3, 8-13 Jun. 2003 uses calibration signals from multiple calibration sources mounted in front of the array. The near-field signals from the calibration sources are monitored, and changes in the signal amplitude and/or phase indicate the need to correct the element excitation. A similar technique is used in Meyer, R. X.; “Electronic Compensation for Structural Deformations of Large Space Antennas”, Astrodynamics Proceedings of the Conference, Vail, Colo., Aug. 12-15, 1985. pp. 277-285, where the elements in the array receive a signal from calibration sources mounted on a mast near the aperture. In Lee, E.-A.; Dorny, C. N., “A broadcast reference technique for self-calibrating of large antenna phased arrays,” Antennas and Propagation, IEEE Transactions on, vol. 37, no. 8, pp. 1003-1010, August 1989, Lee and Dorny also present a calibration technique for large arrays that requires the integration of auxiliary calibration beacon signals.
Accordingly, these calibration procedures either 1) do not permit calibration during array operation or 2) require external sources to provide a calibration signal to the radiating elements in the array. It would be desirable to provide a calibration technique without these shortcomings.